As internal combustion engines have increasingly higher performance such as higher power, it is demanded that piston rings have excellent wear resistance and scuffing resistance. Thus, outer peripheral surfaces of piston rings made of cast iron or steel have been subjected to surface treatments such as hard chromium plating, nickel composite plating, nitriding, chromium nitride ion plating and thermal spraying, etc. In diesel engines used under particularly severe conditions, thermal spray coatings of cermets are used, but when combined, for instance, with cylinder liners of ferrite-rich, soft cast iron (FC200 to 300) having a tensile strength of 300 MPa or less, the cylinder liners disadvantageously suffer from large wear near top dead points. Accordingly, it is required that thermal spray coatings formed on piston rings have little attackability on mating members with excellent wear resistance and scuffing resistance.
JP 3-172681 A discloses a dense thermal spray coating with good wear resistance, scuffing resistance and peeling resistance, which is formed by plasma-spraying of a mixed powder of Cr3C2 and Ni—Cr alloy in an inert gas atmosphere under reduced pressure. JP 8-210504 A discloses a piston ring having a thermal spray coating formed at least on its outer peripheral surface by high-velocity oxygen fuel (HVOF) spraying, the thermal spray coating comprising a first layer as an undercoat and a second layer as a topcoat, the first layer comprising 20 to 80% by mass of Cr3C2 and the balance being a Ni—Cr alloy, and the second layer being made of a cobalt-based or nickel-based sliding material comprising Mo and Cr as main components. Though these thermal spray coatings are considerably improved in wear resistance, scuffing resistance and peeling resistance, their attackability on mating members has not been sufficiently lowered yet.
In conventional thermal spray coatings of chromium carbide/Ni—Cr alloy, pulverized powder having a particle size of several tens of microns is used as thermal spray powder. However, the pulverized powder of a Ni—Cr alloy is thrown against a substrate surface by thermal spraying, forming a flat shape as large Ni—Cr alloy regions as 20 to 40 μm. Thus, the resultant thermal spray coating has an uneven microstructure. When such thermal spray coating is used on a piston ring, the Ni—Cr alloy regions wear first, and the remaining chromium carbide-rich regions abrade mating members. Also, because the coating structure is uneven, the surface roughness of the thermal spray coating cannot be reduced to a desired level or less even by grinding, resulting in wearing a mating cylinder liner. Further, because there are locally extremely hard portions composed only of chromium carbide, an inlaid piston ring having a layer thermally sprayed in a center groove on an outer peripheral surface disadvantageously have steps on groove edges after finish-working of the outer peripheral surface.